This invention relates to a power metal-oxide-semiconductor integrated circuit (MOS IC) and, more particularly, to a protective circuit employed in such an IC for protecting a power MOS transistor from thermal destruction.
Since a power MOS IC consumes a large power, it operates under a condition of a relatively high operating temperature. It is therfore required to protect the IC, a power transistor in particular, from a thermal destruction. For his purpose, the power MOS IC employs an overheat protective circuit for the power transistor.
Referring to FIG. 6, such a conventional overheat protective circuit comprises a first voltage line 50 receiving a power voltage Vcc, a reference voltage generator 2 generating and supplying on a voltage line 55 a reference voltage BGR2 stabilized against a change in an operating temperature, another reference voltage generator 1 connected between the first voltage line 50 and the second voltage line 55 and generating at its output node 60 a reference voltage BGR1 which is also stabilized against the operating temperature, a temperature detective circuit 3 connected between the first voltage line 50 and the second voltage line 55 and generating at its output node 65 a detection voltage OT which is changed in accordance with the operating temperature, and a comparator 4 comparing the reference voltage BGR 1 with the detection voltage OT. The output from the comparator 4 is supplied to the gate of a shunt MOS transistor 6 of an N-channel type connected between the gate and source of an N-channel output power MOS transistor 7 as a transistor to be protected. This transistor is connected between the first voltage line 50 and an output terminal OUT and driven by a drive circuit 5 such as a charge pump circuit.
The operation will be described below. When this circuit operates under a normal operating temperature, the detective circuit 3 produces such a detection voltage OT that has a level lower than that of the reference voltage BGR1. Accordingly, the comparator 4 outputs the low level to turn off the shunt MOS transistor 6. The output transistor 7 thereby responds to the signal from the drive circuit 5 to drive a load (not shown) connected to the output terminal OUT. The level of the detection voltage depends on the operating temperature. When the operating temperature reaches a predetermined temperature due to, for example a load short, the detection voltage OT exceeds the level of the reference voltage BGR1. The output voltage of the comparator 4 is thereby changed from the low level to the high level to turn the transistor 6 on. The current path between the gate and source of the output MOS transistor 7 is thus shunt to render the output MOS transistor 7 nonconductive irrespective of the drive signal from the driver circuit 5. In such a manner as described above, the output transistor 7 is protected from thermal destruction.
Turning to FIG. 8, the temperature detective circuit 3 is constructed by a resistor 31 connected between the first voltage line 50 and a first node, a resistor 32 connected between the first node and a second node, a bipolar transistor 33 connected between the first voltage line 50 and the second node and having a base connected to the first node, a plurality of diodes 34 connected in series between the second node and the output node 65, and a current source 35 connected between the output node 65 and the second voltage line 65. Since each diode has a negative temperature coefficient in a forward voltage thereof, the level of the detection voltage OT increases in accordance with the increase in operating temperature.
On the other hand, the reference voltage operator generator 1 is composed of the so-called band gap regulator. Since the band gap regulator is well known in the art, the circuit construction thereof will be omitted.
Under the condition of the typical level of the power supply voltage Vcc, the reference circuit 1 generates the reference voltage BGR1 having a preset level and the detective circuit 3 also generates the detection level having a preset level depending on the operating temperature; however, when the power supply voltage Vcc is lowered due to a heavy load or a change of a power source for generating the power voltage Vcc, each of the levels of the reference voltage BGR1 and the detection voltage OT is also lowered accordingly. In this case, if the lowering in level of the reference voltage BGR1 would be proportional to that in level of the reference voltage OT, the comparator would not produce an erroneous output signal. However, there is in fact no proportional relationship therebetween, as shown in FIG. 7. This is, the reference voltage BGR1 represents a linear change, whereas the detection voltage OT represents a non-linear change. This is based on the difference in circuit construction between the reference voltage circuit 1 and the detective circuit 3. For this reason, the level of the detection voltage OT exceeds that of the reference voltage BGR1 within a range "A" of the power voltage Vcc as indicated in FIG. 7. The comparator 4 thereby changes its output signal from the lower level to the high level within the range A of the power voltage Vcc irrespective of the fact that the operating temperature is below the designed value for protecting the output transistor 7. The transistor 6 is thereby rendered conductive to cause the output transistor 7 to stop driving the load. The error in circuit operation thus take place.